Optical semiconductor device and method of manufacturing same

ABSTRACT

The reliability of a buried hetero-structure semiconductor laser is improved by preventing an increase in oscillation threshold current and a decrease in external differential quantum efficiency in cases where the semiconductor laser is energized continuously under conditions of high temperature and high optical output. An optical semiconductor laser has an optical waveguide structure comprising an n-type cladding layer, an active layer and p-type cladding layers, and a current narrowing/blocking structure comprising a p-type blocking layer and an n-type blocking layer, wherein concentration of hydrogen contained in the p-type cladding layers is higher than concentration of hydrogen contained in the p-type blocking layer.

REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of the priority ofJapanese patent application No. 2007-262253, filed on Oct. 5, 2007, thedisclosure of which is incorporated herein in its entirety by referencethereto.

TECHNICAL FIELD

This invention relates to an optical semiconductor device and method ofmanufacturing the same. More particularly, the invention relates to aburied semiconductor laser and method of manufacturing the same.

BACKGROUND

The smooth development of the Internet, which has expanded at a rapidpace, is being sustained by an increase in the capacity ofcommunications by virtue of optical fiber. The scope of application ofoptical fiber communication networks has broadened to include not onlytrunk line systems but also access systems and subscriber systems.Inexpensive semiconductor lasers capable of withstandinghigh-temperature operation are in demand particularly in FFTH(Fiber-To-The-Home), a system in which optical fiber is connected to thehome of each subscriber. In semiconductor lasers for optical subscribersystems, achieving a low oscillation threshold current, a low drivingcurrent and a low driving voltage is essential for the purpose ofreducing power consumption, and a BH (Buried Hetero) structure generallyis employed. In particular, a PBH (Planar Buried Hetero) laser in whicha pnpn thyristor layer is adopted as a current narrowing/blockingstructure and both sides of the light-emitting portion are buried is inwide use in order to reduce leakage current (reactive current).

As an example, reference will be had to the drawings to describe thestructure and method of manufacturing a buried semiconductor laser on anInP substrate according to the conventional art, which is set forth inPatent Document 1 and Non-Patent Document 2.

FIGS. 10A-10D is a diagram useful in describing a method ofmanufacturing a conventional semiconductor laser, and FIG. 11 is aperspective view of the semiconductor laser.

First, an active layer 22 comprising InGaAsP or the like and a p-typeInP cladding layer 23 a are grown epitaxially on an n-type InP substrate21 successively (FIG. 10A). Next, a dielectric mask 24 comprising SiO₂or the like is formed in the shape of a stripe on the p-type InPcladding layer 23 a and, using this as a mask, the p-type InP claddinglayer 23 a, active layer 22 and n-type InP substrate 21 are etched intoa mesa stripe (FIG. 10B). A current narrowing/blocking structurecomprising a p-type InP blocking layer (buried layer) 25 and n-type InPblocking layer 26 is then grown burying the sides of the mesa stripe(FIG. 10C). After the dielectric mask 24 is removed, a p-type InPover-cladding layer 23 b and a p-type InGaAs contact layer 27 are grownepitaxially (FIG. 10D).

By subjecting the wafer (FIG. 10D) that has thus undergone epitaxialcrystal growth to an electrode formation process for a p-side electrode28 and n-side electrode 29, the semiconductor laser of FIG. 11 isobtained.

In general, Zn on the order of 1×10¹⁸ cm⁻³ is used as the dopant(acceptor) in the p-type InP blocking layer 25 and p-type InPover-cladding layer 23 b, and Si on the order of 1×10¹⁸ cm⁻³ is used asthe dopant (donor) in the n-type InP blocking layer 26.

It should be noted that Patent Document 2 discloses a method ofmanufacturing a semiconductor laser in which a rise in resistance of acladding layer due to diffusion of hydrogen is prevented while theintroduction of a Group-V defect in the cladding layer and active layeris prevented by carrying out a temperature lowering process, whichfollows the formation of a heterostructure, in an atmosphere thatincludes a hydrogen-containing compound serving as a Group-V rawmaterial and carrying out a temperature lowering process, which followsthe formation of a p-type contact layer, in an atmosphere that does notinclude a hydrogen-containing compound serving as a Group-V rawmaterial.

[Patent Document 1]

Japanese Patent Kokai Publication No. JP-2007-103581A (FIG. 1)

[Patent Document 2]

Japanese Patent Kokai Publication No. JP-P2002-26458A

[Non-Patent Document 1]

Ikegami, Tsuchiya, Mikami, “Semiconductor Photonic Device Engineering”,Corona Publishing Co., Ltd., Jan. 10, 1995, pp. 202-203

SUMMARY OF THE DISCLOSURE

The entire disclosures of Patent Documents 1 and 2 and Non-PatentDocument 1 are incorporated herein by reference thereto. The followinganalyses are given by the present invention.

The analysis set forth below has been performed by the presentinventors.

A problem set forth below arises in a case where the conventional PBHlaser is used in an application that requires operation at hightemperature and high optical output as in the manner of FTTH.

Specifically, in a case where a conventional PBH laser is operatedcontinuously at a high temperature and a high optical output, anincrease in oscillation threshold current and a decrease in externaldifferential quantum efficiency occur and operating current increases.Since the lifetime of a semiconductor laser is decided by the rate ofincrease in operating current, the operating conditions mentioned abovelead to a shorter lifetime for the semiconductor lasers. SinceInGaAsP/InP-group semiconductor lasers that have come to be used intelecommunications are employed upon being temperature-adjusted to aboutroom temperature (20 to 30 degrees C.), any increase in operatingcurrent has been negligible. However, semiconductor lasers used in FTTHapplications are required to be driven continuously at high temperaturesof 85 to 95 degrees C. or greater and at high outputs of 10 mW orgreater. Under such high temperatures and high output power, shorteningof lifetime due to an increase in operating current can be a seriousproblem.

Accordingly, it is an object of the present invention to improve thereliability of a buried semiconductor laser by preventing an increase inoscillation threshold current and a decrease in external differentialquantum efficiency, which occur when the semiconductor laser is drivenunder conditions of high temperature and bias.

A semiconductor laser (or optical semiconductor device) according to thepresent invention has an optical waveguide structure comprising at leastan n-type cladding layer, an active layer and a p-type cladding layer,and a current narrowing/blocking structure comprising a p-type blockinglayer and an n-type blocking layer, wherein concentration of hydrogencontained in the p-type cladding layer is higher than the concentrationof hydrogen contained in the p-type blocking layer.

Further, it is preferred that the concentration of hydrogen contained inthe p-type cladding layer be two or more times greater than theconcentration of hydrogen contained in the p-type blocking layer.

The Inventors have clarified the mechanism whereby an increase inoscillation threshold current and a decrease in external differentialquantum efficiency occur in a case where a PBH laser is operatedcontinuously under conditions of high temperature and high output power.This mechanism will now be described while referring to thecross-sectional structure of a PBH-type semiconductor laser in FIG. 3.

Metal-Organic Vapor Phase Epitaxy (MOVPE) is used to achieve crystalgrowth in a laser structure. At the time of crystal growth of a p-typeInP current blocking layer 15 and a p-type InP cladding layer 13 b,hydrogen that evolves from phosphine (PH3), which is a decomposedGroup-V raw-material gas, is taken in and remains in the crystal evenafter crystal growth. It is known that hydrogen (H) in the p-typesemiconductor layers compensates for the acceptor (this is referred toas “hydrogen passivation”) and lowers the carrier concentration(acceptor concentration) of the p-type semiconductor. Further, since thehydrogen in the p-type semiconductor is stable in the state of H⁺,migration of hydrogen of the kind shown by arrows in FIG. 3 occurs whena current is passed under conditions of high temperature and high bias(forward bias).

A first hydrogen migration path is path “a” from the p-type InP blockinglayer 15 to the an n-type InP substrate 11, and a second path is a path“b” from the p-type InP cladding layer 13 b to the p-type InP blockinglayer 15.

When migration of hydrogen on path “a” occurs, acceptor compensation(hydrogen passivation) of the p-type InP blocking layer 15 is suppressedand the carrier concentration of the p-type InP blocking layer 15 rises(resistance lowers) in comparison with that before conduction ofcurrent. When migration of hydrogen on path “b” occurs, on the otherhand, acceptor compensation (hydrogen passivation) of the p-type InPblocking layer 15 is promoted and, as a result, the carrierconcentration of the p-type InP blocking layer 15 falls (resistancerises) in comparison with that before conduction of current.

If the carrier concentration of the p-type InP blocking layer 15 is high(the resistance is low), leakage current in a state before reaching thelaser oscillation threshold value increases. Consequently, theoscillation threshold current rises. Conversely, if the carrierconcentration of the p-type InP blocking layer 15 is low (the resistanceis high), then the leakage current in the state before reaching thelaser oscillation threshold is suppressed. As a result, the oscillationthreshold current decreases.

Accordingly, the oscillation threshold current increases or decreasesdepending upon whether the amount of change in concentration of hydrogenin the p-type InP blocking layer 15 before and after conduction ofcurrent is positive or negative. That is, an increase or decrease inoscillation threshold current is decided depending on in which path, aor b, the migration of hydrogen (H⁺) illustrated in FIG. 3 is greater.

Generally, in a PBH laser, the current-blocking breakdown voltage of thepnpn thyristor structure is elevated by raising the doping concentrationin the p-type InP blocking layer 15 within limits that will not have anadverse effect upon the initial characteristics of the laser. On theother hand, by keeping the doping concentration in the p-type InPcladding layer 13 b low within limits that will not cause an excessiverise in resistance, light-absorption loss is suppressed and opticaloutput is increased, in general.

As a result, the following holds for the carrier concentration of thep-type layers: [p-type InP blocking layer 15]≧[p-type InP cladding layer13 b], and the following also holds for the concentration of hydrogen(H⁺) that compensates for the p-type carrier (acceptor): [p-type InPblocking layer 15]≧[p-type InP cladding layer 13 b]. This means that theamount of migration of hydrogen (H⁺) due to conduction (passage) ofcurrent is greater for path “a” than for path “b”. Consequently, when acurrent is passed under conditions of high temperature and high bias(high optical output), the oscillation threshold current rises and thisrise is accompanied by a decline in the external differential quantumefficiency.

Thus, the mechanism whereby oscillation threshold current increases andexternal differential quantum efficiency declines in a case where a PBHlaser is energized under conditions of high temperature and high bias isas set forth above.

The meritorious effects of the present invention are summarized asfollows.

With the optical semiconductor device according to the presentinvention, an increase in threshold current and a decline in externaldifferential quantum efficiency (a rise in operating current) can beprevented even in a state of high temperature and high optical outputdrive, and reliability under operating conditions of high temperatureand high output can be improved.

The reason for this is as follows: When a buried hetero-structuresemiconductor laser having a pn current narrowing/blocking structure isenergized in order to be driven under high temperature and high opticaloutput, the hydrogen that remains in the p-type semiconductor layers(the p-type cladding layer 13 b and p-type blocking layer 15) migratestoward the side of the n-type substrate 11. When the hydrogen in thep-type blocking layer 15 migrates toward the side of the n-type claddinglayer (n-type substrate) 11, as on path “a” in FIG. 3, acceptorcompensation (hydrogen passivation) ascribable to residual hydrogen issuppressed and the carrier concentration of the p-type blocking layer 15rises. On the other hand, since the hydrogen in the p-type claddinglayer 13 b migrates toward the side of the p-type blocking layer 15, ason path “b” in FIG. 3, acceptor compensation (hydrogen passivation)ascribable to hydrogen in the p-type blocking layer 15 is promoted.Accordingly, paths “a” and “b” have opposite effects upon a change inthe carrier concentration of the p-type blocking layer 15.

In the optical semiconductor device according to the present invention,the concentration of hydrogen contained in the p-type cladding layer 13b is higher than the concentration of hydrogen contained in the p-typeblocking layer 15. As a result, the amount of migration of hydrogen ionsthrough the path “b” in FIG. 3 becomes relatively large and a rise inthe carrier concentration of the p-type blocking layer 15 due toconduction (passage) of current can be prevented.

In particular, in a case where the hydrogen concentration of the p-typecladding layer 13 b is made two or more times greater than the hydrogenconcentration of the p-type blocking layer 15, the effect of reducingthe oscillation threshold current by conduction of current becomes morepronounced and the reliability of the optical semiconductor device canbe improved further.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are diagrams useful in describing a method ofmanufacturing an optical semiconductor device according to an exemplaryembodiment of the present invention;

FIG. 2 is a perspective view illustrating the structure of an opticalsemiconductor device according to an exemplary embodiment of the presentinvention;

FIG. 3 is a diagram useful in describing the effects of an opticalsemiconductor device according to an exemplary embodiment of the presentinvention;

FIG. 4 is a dopant concentration profile in an optical semiconductordevice according to exemplary embodiment of the present invention;

FIG. 5 is a hydrogen concentration profile (comparative) in an opticalsemiconductor device according to a conventional art;

FIG. 6 is a hydrogen concentration profile in an optical semiconductordevice according to a first example of the present invention;

FIG. 7 is a crystal-growth characteristic diagram of the opticalsemiconductor device according to the first example;

FIG. 8 is a crystal-growth characteristic diagram of the opticalsemiconductor device according to a second example of the presentinvention;

FIG. 9 is a hydrogen concentration profile in the optical semiconductordevice according to the second example;

FIGS. 10A to 10D are diagrams useful in describing a method ofmanufacturing an optical semiconductor device according to theconventional art;

FIG. 11 is a perspective view illustrating the structure of an opticalsemiconductor device according to the conventional art;

FIGS. 12A and 12B illustrate the results of an ACC test applied to anoptical semiconductor device according to an exemplary embodiment of thepresent invention;

FIG. 13 illustrates the result of an APC test applied to an opticalsemiconductor device according to an exemplary embodiment of the presentinvention; and

FIG. 14 illustrates the result of an APC test (comparative) applied toan optical semiconductor device according to the conventional art.

PREFERRED MODES OF THE INVENTION

An optical semiconductor device according to examples of the presentinvention will now be described in detail with reference to thedrawings. It goes without saying that other examples also fall withinthe scope of the claims as long as they are in line with the gist of thepresent invention.

FIGS. 1A to 1D are diagrams useful in describing a method ofmanufacturing an optical semiconductor device according to an example ofthe present invention, and FIG. 2 is a perspective view illustrating thestructure of an optical semiconductor device according to the example.

An optical semiconductor device according to the present invention hasan optical waveguide structure comprising at least an n-type claddinglayer 11, an active layer 12 and p-type cladding layers 13 a, 13 b, anda current narrowing/blocking structure comprising a p-type blockinglayer 15 and an n-type blocking layer 16, wherein concentration ofhydrogen contained in the p-type cladding layers is higher thanconcentration of hydrogen contained in the p-type blocking layer.

Further, it is preferred that the concentration of hydrogen contained inthe p-type cladding layers 13 a, 13 b be two or more times greater thanthe concentration of hydrogen contained in the p-type blocking layer 15.

Further, the n-type cladding layer 11 and n-type blocking layer 16 maybe n-type InP, and the p-type cladding layers 13 a, 13 b and p-typeblocking layer 15 may be p-type InP.

The method of manufacturing the optical semiconductor device accordingto this example of the present invention includes steps of: forming afirst layer structure, which includes the active layer 12 and firstp-type cladding layer 13 a, on the n-type cladding layer 11; forming amesa structure by mesa-etching part of the first p-type cladding layer13 a, active layer 12 and n-type InP cladding layer 11; burying the mesastructure by forming a second layer structure having a currentnarrowing/blocking structure that includes the p-type blocking layer 15and n-type blocking layer 16; and forming a third layer structure, whichincludes the second p-type cladding layer 13 b, on the first p-typecladding layer 13 a and current narrowing/blocking structure; whereinthe concentration of hydrogen contained in the second p-type claddinglayer 13 b is made higher than the concentration of hydrogen containedin the p-type blocking layer 15.

Further, in the method of manufacture described above, the n-type InPsubstrate 11 and n-type blocking layer 16 may be n-type InP, and thefirst and second p-type cladding layers 13 a, 13 b and p-type blockinglayer 15 may be p-type InP.

Furthermore, the first to third layer structures may be formed byMetal-Organic Vapor Phase Epitaxy (MOVPE).

Further, it is preferred that substrate temperature when supply ofGroup-V material gas is halted at the end of metal-organic vapor-phasegrowth of the second layer structure be higher than substratetemperature when supply of Group-V material gas is halted at the end ofmetal-organic vapor-phase growth of the third layer structure. Note theGroup-V of elements is according to the international periodic table ofIUPAC 1970.

Furthermore, it is preferred that the amount of Group-V material gassupplied from the end of metal-organic vapor-phase growth of the secondlayer structure until the supply of the Group-V material gas is haltedbe less than the amount of Group-V material gas supplied from the end ofmetal-organic vapor-phase growth of the third layer structure until thesupply of the Group-V material gas is halted.

With reference to FIGS. 1A to 1D, the active layer 12 comprising InGaAsPor the like and the p-type InP cladding layer 13 a are grown epitaxiallyon the n-type InP substrate 11 successively (FIG. 1A). Next, adielectric mask 14 comprising SiO₂ or the like is formed in the shape ofa stripe on the p-type InP cladding layer 13 a and, using this as amask, the p-type InP cladding layer 13 a, active layer 12 and n-type InPsubstrate 11 are etched into a mesa stripe (FIG. 1B). Furthermore, acurrent blocking structure comprising a p-type InP blocking layer 15 andn-type InP blocking layer 16 is grown, burying the sides of the mesastripe (FIG. 1C). After the dielectric mask 14 is removed, the p-typeInP over-cladding layer 13 b and a p-type InGaAs contact layer 17 aregrown epitaxially (FIG. 1D).

FIG. 2 is a perspective view illustrating a structure of an opticalsemiconductor device according to this example of the invention.

As shown in FIG. 2, the optical semiconductor device is obtained byforming an n-side electrode 19 and p-side electrode 18 on a wafer thathas been subjected to epitaxial growth in the manner described above.The hydrogen concentration of the p-type InP cladding layer 13 b in thisstructure is set to be higher than that of the p-type blocking layer 15(this shall be referred to as “structure A” below).

Various evaluation tests were conducted using a laser resonator lengthof 300 micrometers having front and rear end faces coated with 30% and90% reflective films. The results are shown below. Further, an elementhaving the conventional (comparative) structure (referred to as“structure B” below) was also fabricated and compared with structure A.In structure B, the hydrogen concentration of the p-type InP claddinglayer 13 b was set to be approximately equal to that of the p-type InPblocking layer 15.

No significant difference in current vs. optical output characteristicor in current vs. voltage characteristic was observed between structureA of the optical semiconductor device according to the example of thepresent invention and structure B of the conventional opticalsemiconductor device. In order to evaluate reliability, an accelerateddegradation test, in which a current is passed under ACC (AutomaticCurrent Control), was conducted under high-temperature, high-biasconditions of 100 degrees C. and 200 mA, which are more severe than theactual driving conditions. When 100 elements having structure A and 100elements having structure B were energized for 50 hours under theconditions of 100 degrees C. and 200 mA, distributions of a change(ΔIth) in oscillation threshold current (Ith) before and afterenergization were obtained and are as shown in FIGS. 12A and 12B.Specifically, FIG. 12A illustrates the Δ Ith distribution with regard tothe elements having structure A according to this example of theinvention, and FIG. 12B illustrates the Δ Ith distribution with regardto the elements having structure B according to the conventional art. Itwill be understood that with the elements having structure B, theaverage of Δ Ith is a large +20% (the oscillation threshold current isrising). With the elements having structure A of the present invention,on the other hand, it will be understood that Ith is decreasing(improving) although the average of ΔIth is −0.5%, which is very small.

Furthermore, in order to investigate long-term reliability, an APC(Automatic Power Control) test was conducted under high-temperature,high-output conditions of temperature 85 degrees C. and optical output15 mW. The APC test is one that adjusts driving current automatically soas to hold optical output constant. Amount of change (ΔIop) in drivingcurrent (Iop) is monitored with respect to driving time.

FIG. 13 illustrates the result of the APC test applied to the elementshaving structure A according to this example of the present invention.After lop drops (improves) slightly at the beginning of energization,there is almost no change and lop remains stable. On the other hand,FIG. 14 (comparative) illustrates the result of the APC test applied tothe elements having structure B according to the conventional art. Thedriving current lop increased with time and a rise in lop of 5 to 10%was observed after elapse of 3500 hr.

It was demonstrated from these test results that when average elementlifetime is calculated under conditions of temperature 85 degrees C. andoptical output 15 mW, structure A (FIG. 13) affords a high reliabilityof at least 300,000 hr, whereas the figure for structure B (FIG. 14) is30,000 hr or less, which is a difference of more than one order ofmagnitude with regard to average element lifetime. The results of theabove-described ACC test and APC test can be explained by the model ofhydrogen migration in the p-type InP layer described earlier inconjunction with FIG. 3, and both tests indicate the effectiveness ofthe optical semiconductor device according to this example of thepresent invention.

FIRST EXAMPLE

An optical semiconductor device according to a first example of thepresent invention will now be described with reference to FIGS. 1 to 7.

An active layer 12, which is an InGaAsP/InGaAsP strained multiplequantum well (strained MQW), and a p-type InP cladding layer 13 a aregrown epitaxially on the n-type InP substrate 11 by MOVPE (FIG. 1A).

Next, a dielectric mask 14 of SiO₂ or the like is formed on the p-typeInP cladding layer 13 a in the shape of a stripe having a width of 1.5micrometer, and the p-type InP cladding layer 13 a, active layer 12 andn-type InP substrate 11 are dry-etched into the shape of a mesa stripeusing the dielectric mask 14 as a mask (FIG. 1B).

Furthermore, the current blocking structure comprising a p-type InPblocking layer 15 and n-type InP blocking layer 16 is grown by MOVPE,thereby burying the sides of the mesa stripe (FIG. 1C). The p-type InPblocking layer 15 has a film thickness of 1.0 micrometer and a carrierconcentration “p” of 1.0×10¹⁸ cm⁻³, and the n-type InP blocking layer 16has a film thickness of 1.0 micrometer and a carrier concentration “n”of 1.0×10¹⁸ cm⁻³.

After the dielectric mask 14 is removed, a p-type InP over-claddinglayer 13 b (having a film thickness of 2.5 micrometer and a carrierconcentration p of 1.0×10¹⁸ cm⁻³) and the p-type InGaAs contact layer 17(having a film thickness of 0.3 micrometer and a carrier concentration pof 8.0×10¹⁸ cm⁻³) are grown epitaxially (FIG. 1D).

A semiconductor laser structure shown in FIG. 2 is obtained by forming ap-side electrode 18 and n-side electrode 19 on the wafer that has beensubjected to the epitaxial growth in the manner described above.

The wafer having the structure of FIG. 1D was subjected to analysis ofdopant and hydrogen concentration by SIMS (Secondary Ion MassSpectrometry). FIG. 4 is a depth-direction distribution of atomicconcentrations of Zn, which is a p-type dopant, and Si, which is ann-type dopant. Here Zn has been detected in the p-type InP blockinglayer 15, p-type InP cladding layer 13 b and p-type InGaAs contact layer17, and Si has been detected in the n-type InP blocking layer 16. Thereis agreement with the set carrier concentration except for Zn of thep-type InP blocking layer 15 and p-type InP cladding layer 13 b. Theatomic concentration of Zn in the p-type InP blocking layer 15 andp-type InP cladding layer 13 b is 1.3×10¹⁸ cm⁻³. The reason why this ishigher than carrier concentration p=1.0×10¹⁸ cm⁻³ is that the acceptorhas been compensated for by the existence of hydrogen (hydrogenpassivation). A hydrogen concentration of 3.0×10¹⁷ cm⁻³, which isequivalent to the difference with respect to carrier concentration, hasbeen confirmed by the SIMS analysis, as illustrated in FIG. 5.

The hydrogen concentration profile of FIG. 5 is for a case where thep-type InP blocking layer 15 and p-type InP cladding layer 13 b are theresult of crystal growth under the same conditions and corresponds tothe conventional structure (structure B).

On the other hand, FIG. 6 is a hydrogen concentration profile in anoptical semiconductor device according to the first example of thepresent invention. The hydrogen concentration of the p-type InP claddinglayer 13 b 4.5×10¹⁷ cm⁻³, which is a higher than that of the p-type InPblocking layer 15.

A method of thus making the hydrogen concentration of the p-type InPcladding layer 13 b higher than that of the p-type InP blocking layer 15will be described with reference to FIG. 7.

FIG. 7 is a graph obtained by investigating the relationship betweensubstrate temperature and concentration of hydrogen that remains in thep-type InP layers. The substrate temperature is that at which supply ofphosphine (PH3), which is a Group-V raw-material gas, attemperature-drop standby following the end of crystal growth of thep-type InP layers by MOVPE is halted. An experiment was conducted uponfixing the amount of flow of PH3 supplied to 150 ccm. When supply of PH3is halted at a high substrate temperature, hydrogen that has been takeninto the crystal is released. On the other hand, since PH3, which is thesupply source of hydrogen, is absent at this time, residual hydrogenconcentration declines. On the basis of this relationship, when thep-type InP blocking layer 15 is grown (formed), the temperature at whichthe supply of PH3 is halted during temperature-drop standby followingthe end of crystal growth is made 450 degrees C., whereas in the case ofthe p-type InP cladding layer 13 b, this temperature is made 350 degreesC. or less. As a result, the hydrogen concentration of the p-type InPcladding layer 13 b can be made higher than that of the p-type InPblocking layer 15.

SECOND EXAMPLE

An optical semiconductor device according to a second example of thepresent invention will now be described with reference to FIGS. 8 and 9.

In the optical semiconductor device according to this example, thehydrogen concentration of the p-type InP cladding layer 13 b is raisedfurther and is made two or more times greater than the hydrogenconcentration of the p-type InP blocking layer 15.

The amount of PH3 to be supplied is increased during thetemperature-drop standby. This is made in addition to the lowering oftemperature at halting of the PH3 supply during temperature-drop standbyfollowing the end of growth in the first example. As a result, theconcentration of hydrogen evolved by decomposition of PH3 can be raisedand the residual hydrogen concentration in the p-type InP layers can beincreased.

FIG. 8 illustrates results obtained by investigating the relationshipbetween amount of flow of PH3 supplied, with temperature at halting ofthe PH3 supply during temperature-drop standby being fixed at 300degrees C., and the residual hydrogen concentration. The hydrogenconcentration of the p-type InP layers rises as the amount of flow ofsupplied PH3 is increased, as illustrated in FIG. 8. A laser structurewas fabricated with the standby amount of flow of PH3 following the endof growth of the p-type InP blocking layer 15 being made 150 ccm and thestandby amount of flow of PH3 following the end of growth of the p-typeInP cladding layer 13 b being made 400 ccm, and the result ofinvestigating the residual hydrogen profile by SIMS analysis is shown inFIG. 9. It will be understood that while the hydrogen concentration ofthe p-type InP blocking layer 15 is 3.0×10¹⁷ cm⁻³, which is the same asin the first example, the hydrogen concentration of the p-type InPcladding layer 13 b has been raised to 6.5×10¹⁷ cm⁻³, which is more thantwice as high.

Electrodes were formed on the wafer thus subjected to crystal growth tothereby obtain the semiconductor laser structure shown in FIG. 2. Thelaser resonator length was made 300 micrometers and the front and rearend faces were coated with 30% and 90% reflective films. An ACC testunder conditions of 100 degrees C., 200 mA and 50 hr and an APC testunder conditions of 85 degrees C., 15 mW and 3500 hr were conducted.

FIG. 12A illustrates the rate of change (Δ Ith) in oscillation thresholdcurrent before and after the ACC test, and FIG. 13 illustrates the timetrend of rate of change (ΔIop) in driving current evaluated in the APCtest. All of the results were excellent, and an estimated averagelifetime of at least 200,000 hours was obtained even underhigh-temperature, high-output conditions of 85 degrees C. and 15 mW.

Although only an InGaAsP/InP-group buried hetero-structure semiconductorlaser has been described in the foregoing examples, the invention is notlimited to this group of materials. As long as the structure is suchthat migration of hydrogen occurs between the p-type blocking layerstructure and the p-type cladding structure, the invention is applicableto any material and structure. Further, the invention is not limited toa semiconductor laser and can be applied to all types of buried opticalsemiconductor devices such as a semiconductor optical amplifier andmodulator-integrated semiconductor laser.

Further, although the description of the present invention has beenrendered based upon the examples, the present invention is not limitedto these examples.

As many apparently widely different examples of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificexamples thereof except as defined in the appended claims.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

1. An optical semiconductor device comprising: an optical waveguidestructure comprising an n-type cladding layer, an active layer and ap-type cladding layer; and a current narrowing/blocking structurecomprising a p-type blocking layer and an n-type blocking layer; whereinsaid p-type cladding layer has a higher concentration of hydrogen thanthe concentration of hydrogen contained in said p-type blocking layer.2. The device according to claim 1, wherein the concentration ofhydrogen contained in said p-type cladding layer is two or more timesgreater than the concentration of hydrogen contained in said p-typeblocking layer.
 3. The device according to claim 1, wherein said n-typecladding layer and said n-type blocking layer are n-type InP, and saidp-type cladding layer and said p-type blocking layer are p-type InP.